Ferromagnetic material and magnetic apparatus employing the ferromagnetic material

ABSTRACT

A ferromagnetic material can be formed in a very small size on the order of an atomic size and is capable of being stably magnetized. The ferromagnetic material comprises basic unit structures each consisting of a first atom (11), a second atom (12) of the same kind as the first atom (11), and a third atom (or atomic group) (13) of the same kind as the first atom (11) or of a kind different from that of the first atom (11). In each of the basic unit structures, the atoms are arranged on a surface of a substrate so that a chemical bond (14) is formed between the first atom or and the third atom or molecule, a chemical bond (14) is formed between the second atom and the third atom or molecule, a chemical bond or an electron path (15) not passing the third atom is formed between the first and the second atom.

BACKGROUND OF THE INVENTION

The present invention relates to a ferromagnetic material having a widerange of application in technical fields, and magnetic apparatusesincluding high-density magnetic recording apparatuses and magneticsensors and employing such a ferromagnetic material.

A ferromagnetic material is a substance having spontaneousmagnetization, i.e., a substance having a finite magnetizationintensity. Sometimes, a ferromagnetic material in bulk does not displayany finite magnetization intensity. In such a state, the interior of theferromagnetic material is divided into a plurality of regions, each ofthe regions displays magnetization of a finite intensity, and thedirections of magnetization of the regions are different from eachother. A small region in which spontaneous magnetization has a fixeddirection is called a magnetic domain.

A ferromagnetic material is applied widely to various magnetic devicesincluding various magnetic recording systems and magnetic sensors.Efforts have been made for the development of various ferromagneticmaterials suitable for different purposes. In the field of magneticrecording, in particular, the reduction in size of magnetic domains andthe realization of recording of a minimum unit by a smallest possiblenumber of magnetic domains are important problems. Although a pluralityof magnetic domains serve as a recording unit in current magneticrecording, it is desirable to use a single magnetic domain as arecording unit when all is said and done, and it is desirable to reducethe size of magnetic domains each for a recording unit.

A method of making a ferromagnetic material having small magneticdomains employing electron beam lithography is disclosed in, forexample, Journal of Applied Physics, Vol. 76, pp. 6673-6675 (1994). Thismethod forms a region of several tens nanometer square of magnetic atomson a nonmagnetic substrate, i.e., a material not displayingferromagnetism. It is reported in this paper that the region displaysferromagnetism in a single magnetic domain in some cases. Magnetic atomsare atoms which display ferromagnetism in single bulk, such as those of3d transition metals including Cr, Mn, Fe, Co and Ni, and lanthanidesincluding Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er and Tm.

A region of a ferromagnetic material of a size on the order of theforegoing size can be formed by depositing the ferromagnetic material ona nonmagnetic substrate with the probe of a scanning tunnelingmicroscope (STM) as mentioned in, for example, Journal of AppliedPhysics, Vol. 76, pp. 6656-6660 (1994).

SUMMARY OF THE INVENTION

Further miniaturization can not be successfully achieved simply byreducing the size of the region of a magnetic material by the directapplication of the foregoing two methods. Reason for it will beexplained in terms of the Stoner's model which is used for explainingordinary bulky ferromagnetic materials, such as Fe, Co and Ni. Asmentioned in Tokyo Daigaku Bussei Kenkyu-jo, "Bussei Kagaku Jiten", pp.198-200, Tokyo Shoseki (1996), the Stoner's model expresses a conditionfor displaying ferromagnetism (Stoner condition) by U×D(Ef)>1, where Uis electron correlation energy or energy of Coulomb repulsion betweenelectrons, and D(Ef) is electronic density of states at Fermi level. Asubstance must have a very large density of states D(Ef) on a Fermisurface to be ferromagnetic. However, if a minute atomic cluster systemor a fine atom wire system is formed by a ferromagnetic material meetingthe Stoner condition, the density of states D(Ef) is reduced greatly bya finite size effect and, consequently, the Stoner condition cannot bemet and it is highly possible that the spontaneous magnetization of thesystem disappears.

Accordingly, a novel idea entirely different from conventional ideas isnecessary to realize a ferromagnetic material which makes possible afurther smaller single magnetic domain.

Accordingly, it is an object of the present invention to provide aferromagnetic material from which spontaneous magnetization does notdisappear even if the magnetic domain is further miniaturized.

A second object of the present invention is to provide a magnetic headcapable of controlling a magnetic field created by a minute magnetichead comprising an atomic group or a fine atom wire formed on a surfaceof a solid by applying voltage to the surface of the solid as contrastedwith a recording system which supplies a current to an electromagneticinduction magnetic head.

A third object of the present invention is to provide amagnetoresistance effect element including a fine wire having a functionsimilar to that of a magnetoresistance device in a spin valve or amagnetic multilayer film (or an artificial super lattice ofmagnetic/nonmagnetic metals) or a function analogous to themagnetoresistance effect of ferromagnetic tunnel junction.

To solve the foregoing problems, the present invention utilizes a factthat the atomic arrangement and the electronic state of a surface of asolid, and an atom or an atomic group (including molecules) on a surfaceof a solid are different from those of a bulk, i.e., a macroscopicobject. Ferromagnetism is displayed by properly arranging atoms on asurface of a substrate. It is a feature of the present invention thatferromagnetism is displayed by using only nonmagnetic atoms. Nonmagneticatoms are those excluding the previously defined magnetic atoms andatoms of rare gases (He, Ne, Ar, Kr, Xe and Rn).

As mentioned above, according to the Stoner's model, the condition fordisplaying ferromagnetism, i.e., the Stoner condition, is expressed byU×D(Ef)>1, where U is electron correlation energy or energy of Coulombrepulsion between electrons, and D(Ef) is electronic density of statesat Fermi level. Therefore, even substances which are nonmagnetic in abulk state can be made to display ferromagnetism if the Stonercondition: U×D(Ef)>1 can be met by properly arranging atoms on a surfaceof a substrate. For example, the appearance of spontaneous magnetizationat an end of a graphite ribbon is theoretically predicted in, forexample, Journal of Physical Society of Japan, Vol. 65, pp. 1920-1923(1996). However, this structure has not been realized as yet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a typical view of a basic unit structure of a ferromagneticmaterial;

FIGS. 1B and 1C are typical views of ferromagnetic structures formed bycascading a plurality of basic unit structures like that shown in FIG.1A;

FIG. 2 is a typical view of the electronic density of statescorresponding to the basic unit of a ferromagnetic material shown inFIG. 1A;

FIG. 3A is a typical plan view of an atomic arrangement in aferromagnetic material in a first embodiment according to the presentinvention;

FIG. 3B is a typical side view of a portion of the atomic arrangementnear a surface taken in the direction of the arrows A in FIG. 3A;

FIG. 3C is a typical sectional view of a portion of the atomicarrangement near a surface taken on line B--B in FIG. 3A;

FIG. 4 is a diagrammatic view of an energy band structure in the atomicarrangement of FIG. 3A;

FIG. 5A is a typical plan view of an atomic arrangement in aferromagnetic material in a second embodiment according to the presentinvention;

FIG. 5B is a typical sectional view of a portion of the atomicarrangement near a surface taken on line A--A in FIG. 5A;

FIG. 6A is a typical plan view of an atomic arrangement in aferromagnetic material embodying to the present invention;

FIG. 6B is a typical sectional view of a portion of the atomicarrangement near a surface taken on line A--A in FIG. 6A;

FIG. 7A is a typical plan view of an atomic arrangement in aferromagnetic material embodying the present invention;

FIG. 7B is a typical sectional view of a portion of the atomicarrangement near a surface taken on line A--A in FIG. 7A;

FIG. 8A is a typical plan view of an atomic arrangement in aferromagnetic material in a third embodiment according to the presentinvention;

FIG. 8B is a typical sectional view of a portion of the atomicarrangement near a surface taken on line A--A in FIG. 8A;

FIG. 9A is a typical plan view of an atomic arrangement in aferromagnetic material in a fourth embodiment according to the presentinvention;

FIG. 9B is a typical side view of a portion of the atomic arrangementnear a surface taken in the direction of the arrows A in FIG. 9A;

FIG. 9C is a typical sectional view of a portion of the atomicarrangement near a surface taken on line B--B in FIG. 9A;

FIG. 10 is a typical plan view of an atomic arrangement in amagnetoresistance effect element embodying the present inventionconstructed by alternately arranging the ferromagnetic structure shownin FIG. 3A and the nonmagnetic structure of shown in FIG. 9A;

FIG. 11 is a typical plan view of an arrangement of a magnetoresistanceeffect element in a modification of the magnetoresistance effect elementshown in FIG. 10;

FIG. 12 is a perspective view of a magnetic recording head for onestorage unit, in a preferred embodiment according to the presentinvention, employing the structure of the ferromagnetic material shownin FIG. 3A;

FIG. 13 is a perspective view of a magnetic recording head for onestorage element, in another preferred embodiment according to thepresent invention, employing the structure of the ferromagnetic materialshown in FIG. 3A;

FIG. 14 is a diagram showing the variation of expected magneticcharacteristics of the magnetic recording heads of FIGS. 12 and 13 withvoltage applied to the gate electrode;

FIG. 15 is a typical plan view of an atomic arrangement in a magneticrecording medium embodying the present invention employing the structureof the ferromagnetic material of FIG. 3A;

FIG. 16 is a typical plan view of an atomic arrangement in a surface ofa magnetic recording head for one byte, which was constructed byemploying the magnetic recording head for one storage unit shown in FIG.13, facing a magnetic recording medium; and

FIG. 17 is a rear view of the magnetic recording head shown in FIG. 16.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the present invention, a ferromagnetic material is formedby arranging basic unit structures each consisting of nonmagnetic atomsor atomic groups arranged on a surface of a nonmagnetic substrate.Nonmagnetic atoms of the same kind or different kinds are arranged on asurface of a nonmagnetic substrate as shown in FIG. 1A by fineprocessing techniques of an atomic level, such as an atomic manipulationtechnique employing a STM. The basic unit structure shown in FIG. 1A hasan atom 11, an atom 12 of the same kind as the atom 11, and an atom 13of the same kind as the atom 11 or of a kind different from that of theatom 11. The atom 13 may be replaced with a molecule, i.e., an atomicgroup. Each atom or each atomic group may be an atom or a moleculeforming the substrate or may be an external atom or an external moleculedisposed on the substrate. The atoms 11 and 12 and the atom (or theatomic group) 13 are arranged so that chemical bonds indicated by solidlines 14 are formed between the atom 11 and the atom (or the atomicgroup) 13 and between the atom 12 and the atom (or the atomic group) 13are chemical bonds, and an electron path indicated by a broken line 15is formed between the atoms 11 and 12. If the basic unit structureincludes an odd number of atoms which do not participate in formingchemical bonds, the basic unit structure has an electronic density ofstates as typically shown in FIG. 2. Although the shape of theelectronic density of states is dependent on the type of the atoms andthe arrangement of the atoms, the density of states has a peak at apoint near the Fermi level. Since the electronic state density has thepeak, the Stoner condition is satisfied and ferromagnetism is displayed.A ferromagnetic material can be constructed by the single base unit orby arranging a plurality of basic unit structures like that shown inFIG. 1A as shown FIGS. 1B or 1C. In the following description of variousferromagnetic materials embodying the present invention, atoms of thesame kind are represented by the same mark, and only the representativeone of the atoms of the same kind is indicated at a reference character.Incidentally, a circle with small dots represents the constituent atomsof a substrate, a circle with parallel oblique lines sloping down to theleft represents atoms placed on a surface of the substrate, and a smallsolid circle indicates terminal atoms or hydrogen atoms, i.e., moleculeadsorbates.

First Embodiment

FIG. 3A is a typical plan view of an atomic arrangement in aferromagnetic material in a first embodiment according to the presentinvention, FIG. 3B is a typical side view of a portion of the atomicarrangement near a surface taken in the direction of the arrows A inFIG. 3A, and FIG. 3C is a typical sectional view of a portion of theatomic arrangement near a surface taken on line B--B in FIG. 3A.

In this embodiment, the (100) surface of a Si substrate, i.e., anonmagnetic substrate, is used. All the dangling bonds of Si atoms 31 onthe surface of the Si substrate are terminated by hydrogen atoms 33 toobtain chemically inactive, stable surface structure, the probe of a STMare held close to the hydrogen-terminated surface of the Si substrate,and one row of dangling Si bonds in a fine line was formed by extractingone row of hydrogen atoms by applying appropriate voltage pulses to theprobe. Since the row of dangling Si bonds is chemically more active thanthe hydrogen-terminated Si surface structure, Ga atoms 32 could be madeto be selectively adsorbed by the row of dangling Si bonds by utilizinga thermal evaporation source. A procedure including the foregoing stepsis the same as that mentioned in Japanese Journal Applied PhysicsLetters, Vol. 35, pp. 1085-1088 (1996).

In this embodiment, the Ga atoms 32 are made to be adsorbed gradually sothat the number of the adsorbed Ga atoms is 1.5 times the number of thedangling bonds as shown in FIG. 3A. In FIG. 3A, lines between the Siatoms 31 and between the Si atoms 31 and the hydrogen atoms 33 indicatechemical bonds. Regions 300 enclosed by broken lines in the surfacestructure thus formed by arranging the atoms by evaporation correspondto the basic unit shown in FIG. 1A. The Si atoms 31, i.e., theconstituent atoms of the substrate, correspond to the atoms 11 and 12 ofthe basic unit structure shown in FIG. 1A. These atoms are theconstituent atoms of the substrate remaining after the terminal hydrogenatoms 33 have been extracted by the foregoing operation. The threenonmagnetic Ga atoms 32 correspond to the atom (or the atomic group) 13shown in FIG. 1A. In FIG. 3A, the atomic group consisting of the threeGa atoms 32 and the constituent atoms 31 of the substrate are chemicallybonded together. An electron path extends between the atoms 31 via theatomic group of the Ga atoms 32 and another electron path extendsbetween the atoms 31 through the substrate. Therefore, this example hasa magnetic domain structure shown in FIG. 1B.

This structure has an energy band structure as shown in FIG. 4, which isknown from first-principles calculation based on a local densityfunctional method. In FIG. 4, a range between Γ and Jy shows energydispersion relation in a direction parallel to the row of the basic unitstructures. To put it differently, this direction is expressed by adirection of electrical conduction in the structure consisting of thebasic unit structures; that is, the rows of magnetic domain in thisstructure are conductive. As is obvious from FIG. 4, the energy band hasa flat section in this direction in the vicinity of Fermi level Ef.Therefore, a peak electronic state density appears at a position nearthe Fermi level as typically shown in FIG. 2. Therefore, the structureis expected to display ferromagnetism. Although the resolution of thecurrent scanning magnetic force microscope (MFM) or the current spinscanning electron microscope (spin SEM) is not fine enough to enable thedirect observation of the surface magnetic domain structure, it isconjectured from the results of scanning tunnel spectroscopicexperiments that the regions adsorbing Ga atoms may be magnetized andthe direction of magnetization may be aligned with the fine line of Gaatoms. The results of experiments based on scanning tunnel spectroscopy(STS) proved that the electronic state density has a peak at a positionnear the Fermi level. The length of the fine line is dependent on thelength of a region from which hydrogen atoms are extracted. The shortestfine line corresponds to the basic unit structure 300 shown in FIG. 3A.It is obvious that long lines can be fabricated by the same method.

Although the constituent atoms 31 of the substrate are Si atoms in thisembodiment, a substrate of a semiconductor, such as Ge or GaAs, or aninsulating material, such as NaCl, may be used. Although the danglingbonds in the surface of the substrate are terminated by hydrogen atoms33 in this embodiment, the dangling bonds can be effectively terminatedby atoms other than hydrogen atoms or by molecules, such as methylgroups. Although the reduction of chemical activity by the terminationof dangling bonds is very effective in facilitating processing, chemicalactivity need not necessarily be reduced. Actually, a structure similarto that shown in FIG. 3A can be formed by a processing method whichmakes the probe of a STM adsorb a small amount of Ga atoms, holds theprobe holding the Ga atoms close to the surface of a substrate andapplies a pulse voltage to the probe to transfer the Ga atoms from theprobe to the surface of the substrate. If the substrate is thusprocessed, the dangling bonds are not terminated by hydrogen atoms andthe arrangement of Ga atoms is somewhat different from that shown inFIG. 3A, but there is not any hindrance to displaying ferromagnetism.

The nonmagnetic atoms 32 may be atoms of a trivalent metal that belongsto group III in the periodic table of elements, such as B, Al, In or Tl,or those of a plurality of kinds of metals instead of Ga atoms. Forexample, a structure similar to that shown in FIG. 3A and capable ofdisplaying ferromagnetism can be constructed by forming a row ofdangling bonds on a hydrogen-terminated Si substrate, depositing anumber of Ga atoms equal to the number of dangling bonds on the Sisubstrate, and depositing Al atoms equal to half the number of danglingbonds on the Si substrate.

A ferromagnetic material can be produced by using nonmagnetic atoms of ametal of a valence other than those of a trivalent metal. For example, astructure formed by depositing a number of Ca atoms, i.e., bivalentatoms, equal to the number of dangling bonds on a substrate anddepositing a number of Ga atoms equal to half the number of danglingbonds displays ferromagnetism. The arrangement of the atoms on thesurface of the thus processed substrate is not necessarily the same asthat shown in FIG. 3A.

It is essential that the structure has basic units corresponding to thestructure shown in FIG. 1A, and each basic unit structure has an oddnumber of electrons which does not take part in chemical bonding. Morespecifically, the arrangement of the atoms may be dependent on the kindsand the numbers of the atoms and the surface structure of the substrate.

It is effective in protecting the ferromagnetic structure to cover thesurface of the substrate with an insulating material or a semiconductorso that such ferromagnetic material may not be exposed on the surface ofthe substrate after constructing the ferromagnetic structure on thesubstrate.

Although the ferromagnetic material in this embodiment is producedwithout using any magnetic atoms at all, the ferromagnetic material maycontain magnetic atoms as an impurity within or in the vicinity of theferromagnetic structure, provided that the ferromagnetic material hasbasic unit structures corresponding to that shown in FIG. 1A and eachbasic unit structure has an odd number of electrons which do not takepart in chemical bonding.

Second Embodiment

Referring to FIGS. 5A and 5B, a ferromagnetic material in a secondembodiment according to the present invention is constructed by using a(111) surface of a hydrogen-terminated Si substrate without forming arow of dangling bonds.

The (111) surface of the hydrogen-terminated Si substrate is kept at atemperature of 80 K, Ga atoms 32 are deposited on the surface of the Sisubstrate, the Ga atoms 32 are moved with the probe of a STM to form astructure as shown in FIGS. 5A and 5B. In this structure, componentscorresponding to the atoms 11 and 12 and the atom (or the atomic group)13 of the structure shown in FIG. 1A are nonmagnetic atoms 32, i.e., Gaatoms. The constituent atoms 31 of the substrate need not necessarily beSi atoms, the nonmagnetic atoms 32 need not necessarily be Ga atoms, andthe terminating atoms (or molecules) 33 need not necessarily be hydrogenatoms (or molecules). The second embodiment, similarly to the firstembodiment, may employ various kinds of atoms other than the foregoingatoms.

A structure shown in FIGS. 6A and 6B is constructed by arranging threestructures each being similar to the structure shown in FIGS. 5A and 5Bin three parallel rows. The principle of magnetization of the structureshown in FIGS. 6A and 6B is the same as that of the structure shown inFIGS. 5A and 5B, and the same structure is able to form a magneticdomain of a large area.

A structure shown in FIGS. 7A and 7B is similar to that shown in FIGS.6A and 6B. The structure shown in FIGS. 7A and 7B is constructed byarranging two rows of magnetic domain in parallel to each other with theadjacent basic unit structures on the two rows of magnetic domainsharing two nonmagnetic atoms 32. The principle of magnetization of thestructure shown in FIGS. 7A and 7B is the same as that of the structureshown in FIGS. 5A and 5B, and the same structure is able to form amagnetic domain of a large area. When arranging a plurality of basicunit structures of the magnetic domain shown in FIG. 1A, the basic unitstructures need not necessarily be arranged on a straight line as shownin FIGS. 1B or 1C, but may be arranged in a zigzag arrangement or acircular arrangement resembling a circular arc. The basic unitstructures need not necessarily be arranged in a linear arrangement, butmay be arranged two-dimensionally as shown in FIGS. 5A and 5B, FIGS. 6Aand 6B or FIGS. 7A and 7B. FIGS. 5A, 6A and 7A are plan views of thestructures of the ferromagnetic materials embodying the presentinvention formed by arranging atoms, and FIGS. 5B, 6B and 7B are typicalsectional views of portions of the structures near the surface taken online A--A in FIGS. 5A, 6A and 7A.

It is effective in protecting the foregoing structure to cover thesurface of the substrate with a protective means, as was mentioned inthe first embodiment. The structure, similarly to that of the firstembodiment, is capable of displaying ferromagnetism even if the samecontains magnetic atoms as an impurity.

Third Embodiment

FIG. 8A is a plan view of an atomic arrangement in a ferromagneticmaterial in a third embodiment according to the present invention, inwhich basic unit structures of a magnetic domain are arranged in anarrangement corresponding to that shown in FIG. 1C. FIG. 8B is a typicalsectional view of a portion of the atomic arrangement near a surfacetaken on line A--A in FIG. 8A.

The ferromagnetic material in this embodiment can be produced byextracting hydrogen atoms on a row perpendicular to rows of Si dimers onthe surface of a (100) surface of a hydrogen-terminated Si substrate,bringing a STM probe holding Ga atoms close to the surface of the Sisubstrate, and applying an appropriate pulse voltage to the probe.Measurement by STS showed that the electronic state density in astructure shown in FIG. 8A has a peak at a position near the Fermilevel, and it was inferred from this measurement that the structuredisplays ferromagnetism. In this structure, the constituent atoms 31 ofthe Si substrate, i.e., Si atoms, correspond to the atoms 11 and 12 ofthe structure shown in FIG. 1A, and the nonmagnetic atoms 32, i.e., Gaatoms, correspond to the atoms (or atomic groups) 13 of the structureshown in FIG. 1A. Therefore, it is obvious that the basic unitstructures of the structure shown in FIG. 8A are arranged in the samearrangement as that shown in FIG. 1C. This embodiment, similarly to theforegoing embodiments, the constituent atoms 31 of the substrate, thenonmagnetic atoms 32 and the terminating atoms (or molecules) need notnecessarily limited to Si atoms, Ga atoms and hydrogen atoms(molecules), respectively.

Fourth Embodiment

Since the magnetic domain of the basic unit structures shown in FIG. 1Ais ferromagnetic and conductive as mentioned above, the rows of magneticdomains of the structure shown in FIG. 3A is ferromagnetic andconductive.

FIG. 9A is a plan view of a structure of a nonmagnetic material quiteanalogous with that shown in FIG. 3A but differs from the latter in thearrangement of nonmagnetic atoms 33. This structure does not have a flatportion in a energy band in the vicinity of the Fermi level Ef as shownin FIG. 4, and the electronic state density does not have any peak astypically shown in FIG. 2 at a position near the Fermi level. Thestructure shown in FIG. 9A is conductive and is a nonmagnetic fine line.FIG. 9B is a typical side view of a portion of the atomic arrangementnear a surface taken in the direction of the arrows A in FIG. 9A, andFIG. 9C is a typical sectional view of a portion of the atomicarrangement near a surface taken on line B--B in FIG. 9A.

FIG. 10 shows a typical plan view of the structure of amagnetoresistance effect element constructed by alternately arrangingferromagnetic regions 43 of fine conductive lines of a structure likethat shown in FIG. 3A and a nonmagnetic region 44 of conductive finelines of a structure like that shown in FIG. 9A. Atoms of the structures43 and 44 shown in FIG. 10 correspond to those of the structure shown in3A and those of the structure shown in FIG. 9A, respectively. In thisembodiment, the length of a nonmagnetic region between the twoferromagnetic regions is 12 Å to couple the two ferromagnetic regionsantiferromagnetically. In this magnetoresistance effect element, theferromagnetic region 43, the nonmagnetic region 44 and the ferromagneticregion 43 are cascaded on the surface of a semiconductor substrate. Anelectric current flows through the magnetoresistance effect element whena voltage is applied across the opposite ends of the magnetoresistanceeffect element, and the intensity of the current varies according to theintensity of an external magnetic field applied to the magnetoresistanceeffect element. Therefore, a magnetism-detecting head employing themagnetoresistance effect element of this embodiment can be used forreading information from a magnetic recording medium.

A fine line of atoms disclosed in, for example, U.S. Pat. No. 5,561,300or U.S. Pat. No. 5,694,059 may be used for applying a voltage to thefine line of this embodiment or for sending out a signal.

Although this embodiment employs a fine line of the structure containingGa atoms shown in FIGS. 3A and 9A in the ferromagnetic regions 43 andthe nonmagnetic region 44, any suitable structure of other atoms ormolecules may be used instead of the fine line of the structurecontaining Ga atoms, provided that the structure is nonmagnetic andconductive and is capable of antiferromagnetically coupling the twoferromagnetic regions.

In the magnetoresistance effect element in this embodiment, theferromagnetic regions 43 are 10 Å in length and the nonmagnetic region44 is 9 Å in length. Therefore, the ferromagnetic regions 43 which issuperparamagnetic at a room temperature was ferromagnetic at a lowtemperature of 2.1 K.

Fifth Embodiment (Embodiment of 3196037843)

FIG. 11 shows the structure of a magnetoresistance effect elementconstructed by alternately arranging ferromagnetic regions 43 of fineconductive lines of a structure like that shown in FIG. 3A and anonmagnetic region 54 which resembles conductive fine lines of thestructure shown in FIG. 9A. The region 54 of the structure resemblingthat of the nonmagnetic, conductive fine lines shown in FIG. 9A is thesame as the structure shown in FIG. 9A except that the number of Gaatoms included in each basic unit structure of the nonmagnetic region 54of the magnetoresistance effect element of FIG. 11 is less than that ofGa atoms included in each basic unit structure of the nonmagnetic region44 of the magnetoresistance effect element of FIG. 9A by one. Thestructure shown in FIG. 11, similarly to that shown in FIG. 9A, isnonmagnetic, and this fine line is nonconductive. The atoms of thestructure shown in FIG. 11 are the same as those previously describedwith reference to FIGS. 3A and 9A.

Measurement of the region 54 by scanning tunnel spectroscopy (STS)showed that the nonmagnetic, nonconductive region has an energy gap ofabout 1 eV. The length of the nonmagnetic region is 12 Å to couple thetwo ferromagnetic regions on the opposite sides of the nonmagnetic,nonconductive region antiferromagnetically. In this magnetoresistanceeffect element, the ferromagnetic region 43, the nonmagnetic,nonconductive region 54 and the ferromagnetic region 43 are cascaded ona surface of a semiconductor substrate, and a tunnel current flowsthrough the fine line only when a voltage applied to the opposite endsis higher than a critical voltage. The intensity of the tunnel currentis dependent on the intensity of an external magnetic field applied tothe magnetoresistance effect element. If a magnetic head for readinginformation from a magnetic recording medium is fabricated by using themagnetoresistance effect element of this embodiment, information can beread from a magnetic recording medium by applying a voltage thats causea tunnel current to flow to the magnetic head when necessary.

Although this embodiment employs a fine line of the structure containingGa atoms in the ferromagnetic regions 43 and the nonmagnetic,nonconductive region 54, any suitable structure of other atoms ormolecules may be used instead of the fine line of the structurecontaining Ga atoms, provided that the structure is nonmagnetic andconductive and is capable of antiferromagnetically connecting the twoferromagnetic regions.

A fine line of atoms disclosed in, for example, U.S. Pat. No. 5,561,300or U.S. Pat. No. 5,694,059 may be used for applying a voltage to thefine line of this embodiment or for picking up a signal.

Sixth Embodiment (Embodiment of 319603742)

FIGS. 12 and 13 show very small magnetic devices employing theferromagnetic structure shown in FIG. 3A in preferred embodimentsaccording to the present invention. In each of the very small magneticdevices, a fine line 63 of Ga atoms of the structure shown in FIG. 3A isformed on a (100) surface of a Si substrate. The magnetic device shownin FIG. 12 is fabricated by depositing an Au thin film 64 by evaporationon a surface of a Si substrate 60, attaching a Si substrate 62 of athickness on the order of micrometers to the Au thin film 64 so that its(100) surface is exposed, and forming the fine line 63 of aferromagnetic structure as shown FIG. 3A on the (100) surface. Themagnetic device shown in FIG. 13 is fabricated by forming a fine wire ofa ferromagnetic structure as shown in FIG. 3A on a (100) surface of a Sisubstrate 60, and forming a gate electrode 65 close to fine line of theferromagnetic structure with a small space on the order of micrometersbetween the fine line 63 and the gate electrode 65.

FIG. 14 shows the voltage-magnetization characteristics of the magneticdevices shown in FIGS. 12 and 13. In FIG. 14, the potential of the Authin film 64 or the gate electrode 65 relative to the fine line 63 ismeasured on the horizontal axis, and value of magnetization is measuredon the vertical axis. As is obvious from the voltage-magnetizationcharacteristics, the value M of magnetization of the fine line 63 of Gaatoms varies according to the variation of the voltage Vg applied to thegate electrode 64 or 65 in a fixed range, and the magnetization of thefine line 63 of Ga atoms can be controlled by properly determining thegate voltage Vg. The direction of spontaneous magnetization (spin) isparallel to the fine line 63 of Ga atoms. By a magnetization controlmethod using a gate voltage effect of this structure, a minute magneticrecording spot on the order of several hundreds angstroms can be formedon the surface of a magnetic recording medium, by means of a recordingoperation similar to that of an ordinary bulk magnetic recording head.

Seventh Embodiment

FIG. 15 is a typical plan view of an atomic arrangement in a magneticrecording medium embodying the present invention employing the structureof the ferromagnetic material of FIG. 3A, in which reference charactersare omitted for simplicity. In FIG. 15, the same marks as those used inFIG. 3A represent the same atoms, respectively. Atomic fine linesenclosed by broken lines are ferromagnetic. The atomic fine lines aremagnetized or demagnetized by a magnetic recording head shown in FIG. 12or 13. Information can be read from the recording medium by detectingthe state of magnetization of the atomic fine lines by a magnetic headprovided with the magnetoresistance effect element shown in FIGS. 10 or11.

Eighth Embodiment

FIGS. 16 and 17 are a front view and a rear view, respectively, of aportion of a magnetic recording head for use in combination with arecording medium like that shown in FIG. 15 to be disposed opposite tothe recording medium. In FIGS. 16 and 17, reference numerals 71, 72, . .. , and 78 indicate atomic fine lines of the ferromagnetic structureshown in FIG. 3, and reference numerals 81, 82, . . . , and 88 indicateatomic fine lines forming the magnetoresistance effect element shown inFIG. 10. The atomic fine lines are arranged at intervals equal to thosebetween the longitudinal rows of the magnetic domains of theferromagnetic material of the recording medium shown in FIG. 15 to writeor read one byte of information at a time. As mentioned previously inconnection with FIGS. 12 and 13, the atomic fine lines 71, 72, . . . ,and 78 of the ferromagnetic structure and the atomic fine lines 81, 82,. . . , and 88 forming the magnetoresistance effect elements arearranged in parallel rows spaced a distance on the order of micrometersapart. Terminal pads 91A and 91B are connected to the magnetoresistanceeffect element 81 to connect the magnetoresistance effect element 81 toan external device. Terminal pads 92A and 92B, 93A and 93B, . . . , and98A and 98B are connected to the magnetoresistance effect elements 82,83, . . . , and 88 to connect the magnetoresistance effect elements 82,83, . . . , and 88 to the external device. A terminal pad 900 isconnected to the atomic fine lines 71, 72, . . . , and 78 of theferromagnetic structure. Wiring lines 811, 812, . . . , 711, 721, . . ., and 710 connecting the atomic fine lines to the terminal pads may bewiring lines extending from the front surface to the back surface byusing the atomic fine lines disclosed in U.S. Pat. No. 5,561,300 or U.S.Pat. No. 5,694,059.

Information can be recorded on the recording medium by, for example, thefollowing magnetic recording method. The terminal pad 900 is connectedto a reference potential, the magnetoresistance effect elements 81 to 88are set at appropriate potentials corresponding to information to berecorded on the recording medium. Consequently, some of the atomic finelines 71 to 78 of the ferromagnetic structure are magnetized and therest are not magnetized as described previously in connection with FIG.14, whereby the recording elements of the recording medium aremagnetized according to information to be recorded, in which therecording elements of the recording medium corresponding to themagnetized atomic fine lines are magnetized and those of the samecorresponding to not magnetized atomic fine lines are not magnetized.

Information recorded on the recording medium can be read by, forexample, the following magnetic information reading operation. Theelectrical resistances of the magnetoresistance effect elements 81 to 88are dependent on the intensity of magnetic field applied thereto.Therefore, currents of intensities corresponding to the magneticallyrecorded information flow through the magnetoresistance effect elements81 to 88 when an appropriate voltage is applied to the terminal padsconnected to the magnetoresistance effect elements 81 to 88 and hencethe information can be read by measuring the currents which flow throughthe magnetoresistance effect elements 81 to 88.

As is apparent from the foregoing description, according to the presentinvention, ferromagnetic materials and very small magnetic devices canbe constructed by using atoms of specific kinds in combination andproperly arranging those atoms.

What is claimed is:
 1. A ferromagnetic material comprising basic unitstructures each consisting of three nonmagnetic atoms or molecules onsubstrate material of nonmagnetic atoms; wherein, in each of the basicunit structures, the atoms or molecules are positioned so that achemical bond is formed between a first atom or molecule and a thirdatom or molecule, a chemical bond is formed between a second atom ormolecule and the third atom or molecule, a chemical bond or an electronpath not passing the third atom is formed between the first atom ormolecule and the second atom or molecule, and an electronic density ofstates has a peak at a position near the Fermi level, said ferromagneticmaterial exhibiting ferromagnetism.
 2. A ferromagnetic materialaccording to claim 1, wherein each of the basic unit structures consistsof first and second atoms or molecules of the same kind, and a thirdatom or molecule of the same kind as the first and the second atoms orof a kind different from that of the first and the second atom.
 3. Aferromagnetic material according to claim 1, wherein the threenonmagnetic atoms or molecules are positioned so that an energy banddispersion in the direction of electrical conduction has a flat sectionin the vicinity of the Fermi level.
 4. A ferromagnetic materialaccording to claim 1, wherein the first atom or molecule and the secondatom or molecule are constituent atoms or molecules of a substrate whichincludes the substrate material.
 5. A ferromagnetic material accordingto claim 4, wherein the first and second atoms or molecules are in thesurface of the substrate, and wherein the two chemical bonds, betweenthe first atom or molecule and the third atom or molecule and betweenthe second atom or molecule and the third atom or molecule,respectively, are formed between the atom or an atomic group and firstand second atoms in the surface of the substrate.
 6. A ferromagneticmaterial according to claim 1, wherein the substrate material is asemiconductor or an insulating material.
 7. A ferromagnetic materialaccording to claim 4, wherein the substrate is made of a semiconductoror an insulating material.
 8. A ferromagnetic material according toclaim 1, wherein the substrate material is a semiconductor substrate oran insulating substrate made nonconductive by terminating all danglingbonds in a surface thereof with atoms or molecules.
 9. A ferromagneticmaterial according to claim 4, wherein the substrate is a semiconductorsubstrate or an insulating substrate made nonconductive by terminatingall dangling bonds in a surface thereof with atoms or molecules.
 10. Aferromagnetic material according to claim 1, wherein the substratematerial is a semiconductor material or an insulating material madenonconductive by terminating all dangling bonds in a surface thereofwith atoms or molecules, some of the terminating atoms are removed, andatoms of the same kind or atoms of a plurality of kinds are substitutedfor the removed terminating atoms.
 11. A ferromagnetic materialaccording to claim 4, wherein the substrate is a semiconductor substrateor an insulating substrate made nonconductive by terminating alldangling bonds in a surface thereof with atoms or molecules, some of theterminating atoms are removed, and atoms of the same kind or atoms of aplurality of kinds are substituted for the removed terminating atoms.12. A ferromagnetic material according to claim 1, wherein nonmagneticatoms or atomic groups of different kinds are buried in a nonmagneticsubstance.
 13. A ferromagnetic material according to claim 4, whereinnonmagnetic atoms or atomic groups of different kinds are buried in anonmagnetic substance.
 14. A magnetic device comprising:a ferromagneticmaterial, exhibiting ferromagnetism, comprising basic unit structureseach consisting of three nonmagnetic atoms or molecules on a substratematerial of nonmagnetic atoms, the atoms or molecules being positionedso that a chemical bond is formed between a first atom or molecule and athird atom or molecule, a chemical bond is formed between a second atomor molecule and the third atom or molecule, and a chemical bond or anelectron path not passing the third atom is formed between the firstatom or molecule and the second atom or molecule in each of the basicunit structures, and an electronic density of states has a peak at aposition near the Fermi level; and a conductive material disposed so asto be able to apply an electric field to the ferromagnetic material;wherein an electron spin state is switched between a paramagnetic stateand a ferromagnetic state by the electric field applied to theferromagnetic material by the conductive material.
 15. A magnetic deviceaccording to claim 14, wherein the ferromagnetic material is formed onone surface of a semiconductor or insulating substrate, and theconductive material for applying the electric field to the ferromagneticmaterial is formed on the other surface of the semiconductor orinsulating substrate.
 16. A magnetic device according to claim 14,wherein the ferromagnetic material and the conductive material forapplying an electric field to the ferromagnetic material are formed onthe same surface of a semiconductor or insulating substrate.
 17. Amagnetoresistance effect element comprising:a substrate of nonmagneticatoms; cascaded regions of a ferromagnetic material, exhibitingferromagnetism, comprising basic unit structures each consisting ofthree nonmagnetic atoms or molecules on the substrate, the atoms ormolecules being positioned so that a chemical bond is formed between afirst atom or molecule and a third atom or molecule, a chemical bond isformed between a second atom or molecule and the third atom or molecule,and a chemical bond or an electron path not passing the third atom isformed between the first atom or molecule and the second atom ormolecule in each of the basic unit structures, and an electronic densityof states has a peak at a position near the Fermi level; and cascadedregions of a nonmagnetic material comprising basic unit structures eachconsisting of a plurality of atoms arranged so that the density ofstates does not have any peak in the vicinity of the Fermi level, theposition of one of those atoms being different from that of the atom ofthe basic unit structure of the ferromagnetic material.
 18. Amagnetoresistance effect element comprising:a substrate of nonmagneticatoms; cascaded regions of a ferromagnetic material, exhibitingferromagnetism, comprising basic unit structures each consisting ofthree nonmagnetic atoms or molecules on the substrate, the atoms ormolecules being positioned so that a chemical bond is formed between afirst atom or molecule and a third atom or molecule, a chemical bond isformed between a second atom or molecule and the third atom or molecule,and a chemical bond or an electron path not passing the third atom isformed between the first atom or molecule and the second atom ormolecule in each of the basic unit structures, and an electronic densityof states has a peak at a position near the Fermi level; and cascadedregions of a nonmagnetic material comprising basic unit structures eachconsisting of a plurality of atoms arranged so that the density ofstates does not have any peak in the vicinity of the Fermi level and thebasic unit structure is nonconductive, the number of the atoms of thebasic unit structure being less than that of the atoms of the basic unitstructure of the ferromagnetic material by one.
 19. A ferromagneticmaterial according to claim 4, wherein the substrate material isselected from the group consisting of Si, Ge, GaAs and NaCl.
 20. Aferromagnetic material according to claim 4, wherein each of the basicunit structures has an odd number of electrons which do not take part inchemical bonding.
 21. A ferromagnetic material according to claim 1,wherein the substrate material is selected from the group consisting ofSi, Ge, GaAs and NaCl.
 22. A ferromagnetic material according to claim1, wherein each of the basic unit structures has an odd number ofelectrons which do not take part in chemical bonding.
 23. Aferromagnetic material comprising:an Si substrate with a row of danglingbonds constructed by extracting H atoms along an Si dimer row from theH-terminated (100) surface of the Si substrate; and a molecule of threeGa atoms arranged on the surface of said substrate, wherein chemicalbonds are substantially formed between the molecule and two adjacentdangling bonds only so that an electronic density of states of an areaof the two adjacent dangling bonds and the molecule has a peak at aposition near the Fermi level.
 24. A ferromagnetic materialcomprising:an Si substrate with a row of dangling bonds constructed byextracting H atoms along a perpendicular direction of an Si dimer rowfrom the H-terminated (100) surface of the Si substrate; and a pluralityof Ga atoms arranged along the dangling bonds on the surface of saidsubstrate, wherein chemical bonds are substantially formed between eachof the Ga atoms and each of adjacent two dangling bonds corresponding toeach other only so that an electronic density of states of an area ofthe two adjacent dangling bonds and the Ga atom has a peak at a positionnear the Fermi level.
 25. A ferromagnetic material comprising:an Sisubstrate with an H-terminated (111) surface and without forming anydangling bonds in the surface; and three Ga atoms arranged and adsorbedon each center of triangular three hexagonal unit-cells of the hexagonallattice of said surface, wherein an electronic density of states of anarea of the Ga atoms has a peak at a position near the Fermi level. 26.A ferromagnetic material comprising:an Si substrate with an H-terminated(111) surface and without any dangling bonds in the surface; and threenonmagnetic atoms arranged and adsorbed on each center of triangularthree hexagonal unit-cells of the hexagonal lattice of said surface,wherein an electronic density of states of an area of the nonmagneticatoms has a peak at a position near the Fermi level.